Method for producing high-purity diisobutene

ABSTRACT

The invention relates to a process for preparing high-purity diisobutene by reaction of isobutene or isobutene-containing hydrocarbon mixtures over a solid acidic ion-exchange resin containing sulfonic acid groups whose protons have been partly replaced by metal ions and to the use of the diisobutene.

[0001] The invention relates to a process for preparing high-puritydiisobutene from isobutene or isobutene-containing hydrocarbon mixtures.

[0002] Diisobutene, namely a mixture of 2,4,4-trimethyl-1-pentene and2,4,4-trimethyl-2-pentene, is hydrogenated industrially to produce2,2,4-trimethylpentane. Owing to its high octane number, thishydrocarbon is a prized carburetor fuel component. For this purpose, itis also possible to use diisobutene mixtures comprising other C₈ isomersor hydrocarbons having different numbers of carbon atoms. On the otherhand, use in syntheses requires diisobutene of higher purity. Thus,high-purity mixtures are required for preparing 3,5,5-trimethylhexanalby hydroformylation. This aldehyde can be oxidized to give thecorresponding carboxylic acid which is used for preparing peroxides,lubricants and dryers. Diisobutene is also used for the alkylation ofphenols. The alkylaromatics formed in this way are intermediates for theproduction of detergents.

[0003] The oligomerization of isobutene can be catalyzed by Lewis acids,Brönsted acids or coordination compounds, in particular those of nickel.Such oligomerization reactions form oligomers having different molarmasses, since lower oligomers which have already been formed (C₈-,C₁₂-olefins) can react with isobutene or other oligomers to give highermolecular weight olefins. If the starting materials also containn-butenes, cooligomers can also be present in the product.

[0004] For this reason, the economical preparation of2,4,4-trimethylpentenes (1 and/or 2) by dimerization of isobutenerequires not only a good space-time yield but also a high C₈ selectivityand a high C₈ isomer purity. These parameters can be influenced by thetype of catalyst used and the reaction conditions. The catalyst usedmust therefore catalyze neither skeletal isomerization during C₈formation nor isomerization of the C₈-olefin already formed. Otherwise,the product formed will have only limited suitability for chemicalsyntheses.

[0005] The oligomerization can in principle be carried outhomogeneously, i.e. using catalysts which are soluble in the reactionmixture, or heterogeneously, i.e. using catalysts which are insoluble inthe reaction mixture. The disadvantage of homogeneous processes is thatthe catalyst leaves the reactor together with the reaction products andunreacted starting materials from which it has to be separated, workedup and disposed of or recirculated.

[0006] Most industrial processes therefore employ catalysts which arepresent in a fixed bed so that complicated catalyst separation becomesunnecessary. Most known fixed-bed catalysts belong to one of thefollowing groups:

[0007] a) mineral acids (e.g. sulfuric acid or phosphoric acid) on asupport material (e.g. aluminum oxide or silicon dioxide)

[0008] b) zeolites or other aluminosilicates with or without furthermetal(s), in particular transition metals

[0009] c) acidic ion-exchange resins.

[0010] Mineral acids on supports are not suitable for the preparation ofa high-purity mixture of the two 2,4,4-trimethylpentenes from isobutene,since they also catalyze skeletal rearrangements.

[0011] In EP 0 224 220, oligomerization of butene is carried out over abismuth- and/or lead-doped zeolite catalyst. Here, the C₈ fractioncontains more than 4% of undesired 2,3,4-trimethylpentenes. Theoligomerization of isobutene over an X-ray-amorphous aluminosilicate isdisclosed in EP 0 536 839 A2. Here, a loss of 2,2,4-trimethylpentenes byskeletal isomerization cannot be avoided even at the mild temperaturesof 60-65° C.

[0012] Oligomerization of isobutene over an X-ray-amorphous nickelaluminosilicate is described in WO 93/06926. Here, undiluted isobuteneis reacted at 60° C. The product spectrum shows that the C₈ selectivityis not particularly high. At an isobutene conversion of 15-20%, the C₈selectivity is 85-86%, and at a conversion of 75%, only 72%.

[0013] In EP 0 417 407 A1, shaped bodies made of strongly acidic ionexchangers are used as catalyst for the oligomerization of isobutene.Some of these ion exchangers are treated with acid after theirpreparation in order to achieve an increased acidity. The yield ofdimers of 93-96% is good. However, the composition of the C₈ fraction isnot disclosed.

[0014] The use of moderators, for example methyl tert-butyl ether ortert-butanol, for adjusting the catalyst activity of acidic ion-exchangeresins is found to have an advantageous effect on the product spectrum.The major disadvantages are that the moderator has to be separated fromthe product and that it is difficult to obtain a C₈-olefin mixture whichis free of traces of the moderator.

[0015] U.S. Pat. No. 4,447,668 describes a coupled process in which MTBEis firstly cleaved over an acidic ion exchanger to form high-purityisobutene and methanol. The isobutene obtained in this way canoptionally be oligomerized in a liquid phase over an acidic ion-exchangeresin in the presence of methyl tert-butyl ether (MTBE). MTBE serves assolvent and controls the catalyst activity. Distilling off the MTBEleaves an oligomer which comprises up to 97% of diisobutene. No moredetailed information is given about the oligomerization catalyst used orabout the isomer composition of the C₈ fraction.

[0016] U.S. Pat. No. 5,877,372 describes a process for preparing“isooctane” (hydrogenated diisobutene) from tert-butanol. One step inthis process is the oligomerization of isobutene over an acidicion-exchange resin. To set the desired catalyst activity, the startingmaterial for this step contains 1-30% of tert-butanol and, to increasethe C₈-olefin selectivity, 30-80% of “isooctane”. The reaction mixtureis fractionally distilled to give a top product comprising tert-butanoland unreacted isobutene and a bottom product comprising “isooctane” andthe higher oligomers. Over 90% of the oligomer fraction is diisobutene.This mixture is hydrogenated to give “isooctane” containing a fewpercent of higher molecular weight, saturated hydrocarbons, part ofwhich is recirculated to the dimerization step. Catalysts used arecommercial, acidic ion-exchange resins.

[0017] Separating the C₈-olefins from an oligomer obtained by thisprocess requires costly distillation apparatus, since the boiling pointsof “isooctane” 2,2,4-trimethylpentane (99° C.),2,4,4-trimethyl-1-pentene (100-102° C.) and 2,4,4-trimethyl-2-pentene(102-105° C.) are close together. In addition, the composition of theC₈-olefin fraction prepared by this process is not known.

[0018] GB 2 325 237 A describes a process for preparing adiisobutene-containing mixture, in which isobutene is reacted over anacidic ion-exchange resin in the presence of methanol and methyltert-butyl ether. The reaction is carried out in two reactors connectedin series with intermediate separation of the products after the firstreactor. The product mixture from the two reactors comprises, after thelow boilers have been separated off, up to 90% of dimer, higheroligomers and the methyl ether derived from the dimer. Here too, theobject is to obtain a high-octane component or a precursor forcarburetor fuels. On the other hand, the isolation of high-puritydiisobutene is neither envisaged nor described.

[0019] Since the known processes are not entirely satisfactory in termsof the C₈ selectivity and/or the purity of the C₈ fraction, it is anobject of the invention to develop an improved process for preparing ahigh-purity mixture of the two isomeric 2,4,4-trimethylpentenes (1and/or 2).

[0020] It has surprisingly been found that, in the oligomerization ofisobutene in a liquid phase over an acidic ion-exchange resin containingsulfonic acid groups, the selectivity of C₈-olefin formation and the2,4,4-trimethylpentene content of the C₈ fraction is increased when partof the protons is replaced by metal ions.

[0021] The invention accordingly provides a process for preparinghigh-purity diisobutene by reaction of isobutene or isobutene-containinghydrocarbon mixtures over a solid acidic ion-exchange resin containingsulfonic acid groups whose protons have been partly replaced by metalions.

[0022] Acidic ion-exchange resins are usable catalysts for theoligomerization of isobutene only when they have a certain minimumacidity. Thus, resins containing carboxylic acid groups are not acidicenough and are therefore not suitable as catalysts. Suitable resinscontain sulfonic acid groups. As mentioned above, reaction of isobuteneover sulfonated ion-exchange resins results in formation of by-productsif a regulator is not continually fed in together with the startingmaterial.

[0023] It is known from the literature that the acid strength of ionexchangers containing sulfonic acid groups can be reduced by partial ionexchange (Structure-breaking Effect of Metal Ions influencing theAcidity of an Anhydrous Acid, C. Buttersack, H. Widdecke, J. Klein,Journal of Molecular Catalysis, 40 (1987) 23-25). However, it was notobvious that such a modified ion-exchange resin could be usedadvantageously for the oligomerization of isobutene.

[0024] The process of the invention is carried out using solidsulfonated ion-exchange resins in which from 30 to 90% of the protons ofthe sulfonic acid groups, preferably from 50 to 80%, have been replacedby metal ions. As metal ions replacing the protons, it is possible touse alkali metal, alkaline earth metal, chromium, manganese, iron,cobalt, nickel, zinc and aluminum ions and also ions of the lanthanidegroup. Preference is given to using alkali metal ions, in particularsodium ions, for this purpose. It is also possible for the resin to beloaded with two or more different metal ions.

[0025] Suitable ion-exchange resins are, for example, ones obtained bysulfonation of phenol/aldehyde condensates or of cooligomers of aromaticvinyl compounds. Examples of aromatic vinyl compounds for preparing thecooligomers are: styrene, vinyltoluene, vinylnaphthalene,vinylethylbenzene, methylstyrene, vinylchlorobenzene, vinylxylene anddivinylbenzene. Particular preference is given to using the cooligomersformed by reaction of styrene with divinylbenzene as precursor for thepreparation of ion-exchange resins containing sulfonic acid groups. Theresins can be in the form of gels, macroporous or sponge-like. Stronglyacidic resins of the styrene-divinylbenzene type are sold, for example,under the following trade names: Duolite C20, Duolite C26, Amberlyst 15,Amberlite IR-120, Amberlite 200, Dowex 50, K2611, K 2431.

[0026] The properties of these resins, in particular specific surfacearea, porosity, stability, swelling or shrinkage and ion-exchangecapacity, can be varied by means of the production process.

[0027] The ion-exchange capacity is in the range from 1 to 2, inparticular from 1.5 to 1.9, mol of H⁺ per liter of moist resin(commercial).

[0028] In the process of the invention, preference is given to usingmacroporous resins, for example K 2431. The pore volume is from 30 to 60ml/g, in particular from 40 to 50 ml/g (based on commercial resin moistwith water).

[0029] The particle size of the resin is from 500 μm to 1 500 μm, inparticular from 600 μm to 1 000 μm.

[0030] The particle size distribution can be relatively narrow orrelatively broad. Thus, for example, ion-exchange resins having a veryuniform particle size (monodisperse resins) can be used.

[0031] When using a plurality of reactors, these can be charged withresin of the same particle size or a different particle size (orparticle size distribution).

[0032] In the case of reactors through which the reaction mixture flowsat a high linear velocity, it may be advantageous to use a relativelylarge particle size to reduce the differential pressure, while in thecase of reactors through which the reaction mixture flows at a lowlinear velocity, it may be advantageous to use a smaller particle sizein order to achieve optimal conversion.

[0033] If desired, the ion-exchange resins can be used as shaped bodies,for example cylinders, rings or spheres.

[0034] There are a number of possible ways of preparing the catalystshaving the desired activities. If the ion-exchange resin is in the Hform, protons can be replaced by metal ions. If the resin is present asthe metal salt, it can be treated with acids to replace metal ions byprotons. In principle, this ion exchange can be carried out either inorganic suspension or in aqueous suspension. Here, the ion-exchangeresin is slurried with sufficient liquid for a readily stirrablesuspension to be formed. A solution containing the desired ions is addedthereto. After ion exchange is complete, the partly exchangedion-exchange resin is washed and dried.

[0035] A preferred way of preparing the catalysts used in the process ofthe invention is replacement of protons by metal ions in an aqueousphase.

[0036] In the preparation of the catalyst, the ion-exchange resin issuspended in from one to ten times, in particular from one to threetimes, its volume of a solvent.

[0037] To prepare the solution of the desired ion to be added, it isadvisable to choose a solvent which is miscible with the solvent inwhich the resin is suspended. Use of the same solvent is advantageous.

[0038] The ions with which the resin is to be loaded can be in the formof solutions of acids, hydroxides or salts of organic or inorganicacids. In the case of salts of polybasic acids, it is also possible touse acid salts. It is likewise possible to use compounds containingother organic groups, for example alkoxides or acetylacetonates.

[0039] Ion exchange is carried out while stirring in the temperaturerange from 10 to 100° C., in particular from 20 to 40° C.

[0040] The ion solution is added dropwise over a period of from 0.5 to12 hours, in particular from 1 to 3 hours.

[0041] The exchange time (from the commencement of dropwise addition) isfrom 1 to 24 hours, in particular from 3 to 12 hours.

[0042] After ion exchange, the catalyst is separated from the solution,e.g. by decantation or filtration, and is subsequently, if desired,washed with a solvent. It is advantageous to use the same solvent inwhich the catalyst was suspended.

[0043] The moist catalyst is dried, firstly to make it easier to handle(more free-flowing) and secondly to keep the contamination of theproduct with adhering solvent or its downstream products during thefirst days after starting-up the reactor low. Drying can be carried outunder reduced pressure or in a stream of inert gas, for example in astream of nitrogen. The drying temperatures are from 10 to 120° C., inparticular from 40 to 80° C. The drying times are from 1 to 24 hours,depending on pressure and temperature.

[0044] Catalysts of differing activity can be prepared by theabove-described procedure as a function of the degree of ion exchange,the ion type and the resin. A reactor can contain a mixture of resins ofdiffering reactivity. It is likewise possible for catalysts of differingactivity to be arranged in layers in a reactor. If use is made of morethan one reactor, the individual reactors can be charged with catalystshaving the same activity or different activities.

[0045] As starting material, it is possible to use pure isobutene or anisobutene-containing hydrocarbon mixture containing no furtherunsaturated compounds. When using pure isobutene, it is advisable toreduce the concentration by adding a solvent which is easy to separateoff.

[0046] Industrial mixtures comprising isobutene include, for example,naphtha fractions from refineries, C₄ fractions from FCC plants orsteamcrackers, mixtures from Fischer-Tropsch syntheses, mixtures fromthe dehydrogenation of butanes, mixtures from skeletal isomerization oflinear butenes, mixtures formed by metathesis of olefins or otherindustrial processes.

[0047] Prior to use in the process of the invention, isobutene has to beseparated from the other unsaturated compounds in these industrialmixtures. The isolation of isobutene from the C₄ fraction from asteamcracker is employed worldwide and will be described here by way ofexample. The isobutene is separated off in a process comprisingessentially the following steps: the first step is removal of the majorpart of the butadiene. If butadiene can be readily marketed or is usedin-house, it is separated off by extraction. Otherwise, it isselectively hydrogenated to linear butenes down to a residualconcentration of about 2 000 ppm, as described, for example, in EP 523482. Either method leaves a hydrocarbon mixture (raffinate I orhydrogenated crack C4) comprising the saturated hydrocarbons n-butaneand isobutane together with the olefins isobutene, 1-butene and2-butene. Isobutene can be separated from this mixture by reaction withmethanol to form methyl tert-butyl ether (MTBE) or with water to formtert-butanol (TBA). Both MTBE and TBA can, in a reversal of theirformation, be cleaved back into isobutene and methanol or water, asdescribed, for example, in DE 100 20 943.

[0048] Optionally, raffinate I, hydrogenated crack C₄ or a hydrocarbonmixture of similar composition can be hydroisomerized in a reactivecolumn. In this case, a mixture of isobutene and isobutane can beobtained as product from the top.

[0049] The process of the invention can be carried out in batch reactorsor preferably continuously operating reactors as are customarily usedfor solid/liquid contact reactions. When using continuously operatingflow reactors, it is usual but not absolutely necessary to employ afixed bed. If a fixed-bed flow reactor is used, the liquid can flowupward or downward. Downflow of the liquid is usually preferred.Furthermore, the reactor can be operated with product recirculation orin a single pass.

[0050] When using tube reactors, the ratio of length to diameter of thecatalyst bed can be varied, either by means of the geometric dimensionsof the reactor or by means of its fill level. It is thus possible toachieve different empty tube velocities at a given quantity of catalystand LHSV. Reactors in which part of the reaction mixture isrecirculated, preferably after separating off the oligomers, can beoperated at empty tube velocities of from 1 to 30 m/s, in particularfrom 2 to 20 m/s, very particularly preferably from 4 to 10 m/s. Inreactors operated in a single pass, the empty tube velocities can be inthe range from 0.1 to 20 m/s, in particular in the range from 0.8 to 8m/s.

[0051] Owing to the reduced catalyst activity resulting from ionexchange, it is possible to employ lower velocities than in the case ofan ion-exchange resin which has not been subjected to replacement ofprotons, since temperature peaks can be avoided more readily because ofthe lower activity.

[0052] In the case of reactors operating using product recirculation,preferably after the products have been separated off, the spacevelocity over the catalyst (LHSV) is from 0.5 to 15 h⁻¹, in particularfrom 1 to 10 h⁻¹, very particularly preferably from 2 to 5 h⁻¹. In thecase of reactors operated in a single pass, the LHSVs are in the rangefrom 1 to 50 h⁻¹, in particular from 5 to 30 h⁻¹.

[0053] The number of reactors connected in series in the process of theinvention is in the range from 1 to 10, preferably from 1 to 4.

[0054] In a preferred process variant, the first reactor is operatedwith product recirculation and the subsequent reactors are operated in asingle pass.

[0055] Each reactor can be operated adiabatically, polytropically orvirtually isothermally. “Virtually isothermally” means that thetemperature at any point in the reactor is not more than 10° C. higherthan the temperature at the reactor inlet.

[0056] The temperatures at which the reactors are operated are in therange from 20 to 120° C., preferably from 40 to 100° C. The temperatureis dependent on the activity of the catalyst (for example degree of ionexchange).

[0057] The reaction of the invention can be carried out at a pressureequal to or higher than the vapor pressure of the hydrocarbon feedmixture at the respective reaction temperature, preferably at a pressurebelow 40 bar. To avoid vaporization problems in the reactors, thepressure should be 2-4 bar higher than the vapor pressure of thereaction mixture.

[0058] The total conversion of isobutene depends on the type and amountof catalyst used, the prevailing reaction conditions and the number ofreaction stages. For economic reasons, the isobutene conversion is keptin the range from 50 to 100%, preferably from 90 to 100%. In addition,it is advantageous to use hydrocarbons having an isobutene content ofnot less than 5%, preferably not less than 10%, in order to achieve ahigh space-time yield and a high C₈-olefin selectivity.

[0059] To achieve a very high selectivity of C₈-olefin formation, it isadvisable to limit the concentration of oligomers, in particularC₈-olefins, in each reactor. Their concentration is in the range from 0to 50%, in particular in the range from 0 to 30%. The concentration ofoligomers can be limited by selection of operating parameters such astemperature or residence time. A further possibility is to keep theconcentration of isobutene at the reactor inlet below 50%, in particularbelow 30%, by addition of a diluent. It is advantageous to use a diluentwhich is present in the starting material and can be recovered from thereaction mixture after partial or virtually complete reaction of theisobutene. A preferred embodiment of the process is therefore toseparate the mixture leaving the reactor into a fraction comprising theoligomers and a second fraction comprising the diluent(s) and anyunreacted isobutene. Part of the diluent together with the isobutenepresent therein is recirculated to the same reactor or a reactorupstream thereof and the other part is introduced into a downstreamreactor or is worked up. In the case of a plurality of reactorsconnected in series, it is also possible for more than one oligomerseparation step to be present. The oligomers which have been separatedoff (from one or more separation units) are separated into C₈-olefins,C₁₂-olefins and higher oligomers in a further distillation step.

[0060] If desired, the oligomerization can be carried out in a reactivedistillation column containing the catalyst resin which has beensubjected to ion exchange. Here, the abovementioned temperature andpressure ranges apply. The oligomer mixture is obtained as bottomproduct. The top product comprises solvent and any unreacted isobutene.The work-up of these streams is carried out as described above.

[0061] It is also possible to carry out the process of the invention ina plant comprising one or more reactor(s) and a reactive distillationcolumn.

[0062] The high-purity mixture of the two 2,4,4-trimethylpentene isomersprepared by the process of the invention can be hydroformylated toproduce 3,5,5-trimethylhexanal. This aldehyde can be oxidized to thecorresponding carboxylic acid or be hydrogenated to form thecorresponding alcohol. 3,5,5-Trimethylhexanoic acid is used for thepreparation of peroxides, lubricants and dryers. Furthermore,diisobutene is used for the alkylation of phenol or phenol derivatives.The alkylaromatics formed in this way are intermediates for theproduction of detergents. In addition, diisobutene is used for thealkylation of aromatic amines.

[0063] The process of the invention has the following advantages:

[0064] at virtually 100% conversion, the yield of2,4,4-trimethylpentenes is from 80 to 85%. In addition, from 15 to 20%of higher oligomers, of which about 10% are C₁₂-olefins, are formed. TheC₈ fraction has a 2,4,4-trimethylpentene content of over 99.7%, usuallyover 99.8%, and can, owing to its high purity, be used as startingmaterial for chemical syntheses.

[0065] It is not necessary to add an auxiliary to control the productquality, which simplifies the process.

[0066] Since no auxiliaries are added, it is not necessary to separatethem or their downstream products from the reaction mixture, inparticular from the target products. This firstly saves costs for theauxiliaries and secondly simplifies separation. Since the productprepared according to the invention contains no traces of an extraneoussubstance (moderator), subsequent reactions are not adversely affected.

[0067] The following examples illustrate the invention but do notrestrict its scope which is defined by the claims.

[0068] The preparation of the partially neutralized ion-exchange resinswas carried out by reacting the acidic ion-exchange resin, suspended inwater, with the calculated amount of an aqueous alkali metal hydroxidesolution, as described by way of example in example 1.

EXAMPLE 1

[0069] Preparation of a Partially Neutralized Catalyst, Setting of theAcid Capacity

[0070] The ion exchanger used (Amberlyst 15 from Rohm & Haas) had anoriginal acid capacity of 1.43 mol of H⁺/l. To set the desired activity,50% of the acid centers were neutralized.

[0071] For this purpose, 1 000 ml of the ion-exchange resin wereslurried in 1 000 ml of deionized water and, while stirring vigorously,a solution of 28.6 g of sodium hydroxide (0.715 mol) in 500 ml ofdeionized water were added dropwise in the temperature range from 20 to40° C. over the period of one hour. The mixture was stirred for another5 minutes and the ion-exchange resin was then washed three times with 1000 ml each time of deionized water so that it was neutral. Thesubsequent capacity measurement on the partially neutralized ionexchanger gave a value of 0.715±0.03 mol of H⁺/l. The catalyst was driedat 70° C. for 15 hours.

[0072] The oligomerization experiments (examples 2 to 5) were carriedout in a jacketed laboratory tube reactor having a length of 2 m and aninternal diameter of 2 cm. The temperature of the reactor could beregulated by means of a heat transfer fluid which was pumped through thereactor jacket. In all experiments, an isobutane/isobutene mixture wasoligomerized at 23 bar.

EXAMPLE 2

[0073] Oligomerization Using a Catalyst Which has not Been PartiallyNeutralized (Comparative Example) Catalyst: Amberlyst 15 Jacket temp. (°C.): 40 Feed mixture isobutane (% by weight) 83 isobutene (% by weight)16 n-butane (% by weight) 1 LHSV (h⁻¹) 13 Isobutene conversion (%) 99.7Selectivities (%) diisobutene 19 triisobutene 61 higher oligomers 192,4,4-Trimethylpentenes in the C₈ 67 (% by weight)

[0074] Owing to the high acid strength and capacity, the conversion isvery high and the selectivity to dimers is very poor. Owing totransalkylation of the C₈-olefins during or after their formation, thedesired trimethylpentenes make up only part of the C₈-olefins formed.

EXAMPLE 3

[0075] Oligomerization Using Partially Neutralized Catalyst (Accordingto the Invention) Amberlyst 15; 50% of H⁺ Catalyst: replaced by Na⁺Jacket temp. (° C.): 40 Feed mixture isobutane (% by weight) 84isobutene (% by weight) 15 n-butane (% by weight) 1 LHSV (h⁻¹) 13Isobutene conversion (%) 78 Selectivities (%) diisobutene 63triisobutene 33 higher oligomers 3 2,4,4-Trimethylpentenes in theC₈ >99.9 (% by weight)

[0076] Comparison with example 2 shows that although the partialneutralization reduces the conversion, it improves the selectivity todimers under comparable reaction conditions.

[0077] The diisobutene is formed in high isomeric purity.

EXAMPLE 4

[0078] Oligomerization Using Partially Neutralized Catalyst (Accordingto the Invention) Amberlyst 15; 80% of H⁺ Catalyst: replaced by Na⁺Jacket temp. (° C.): 110 Feed mixture isobutane (% by weight) 0isobutene (% by weight) 100 LHSV (h⁻¹) 4 Isobutene conversion (%) 67Selectivities (%) diisobutene 82 triisobutene 17 higher oligomers <12,4,4-Trimethylpentenes in the C₈ >99.9 (% by weight)

[0079] Replacement of 80% of the protons by sodium ions results in asignificant reduction in the reaction rate, so that more severeconditions (higher temperature and/or higher isobutene concentration)are necessary for an industrially acceptable reaction rate. However, aselectivity to dimers better than that in example 3 is obtained despitethe higher temperature.

EXAMPLE 5

[0080] According to the Invention Amberlyst 15; 50% of H⁺ Catalyst:replaced by K⁺ Jacket temp. (° C.): 100 Feed mixture isobutane (% byweight) 45 isobutene (% by weight) 54 n-butane (% by weight) 1 LHSV(h⁻¹) 2 Isobutene conversion (%) 13 Selectivities (%) diisobutene 94triisobutene 5 higher oligomers (% by weight) <1 2,4,4-Trimethylpentenesin the C₈ >99.9 (% by weight)

[0081] This example shows that more severe reaction conditions also makeit possible to dimerize isobutene when H⁺ is partly replaced by K⁺.Compared to the other examples, a very greatly improved selectivity todimers is obtained.

1. A process for preparing high-purity diisobutene by reaction ofisobutene or isobutene-containing hydrocarbon mixtures over a solidacidic ion-exchange resin containing sulfonic acid groups whose protonshave been partly replaced by metal ions.
 2. The process as claimed inclaim 1, wherein from 30 to 90% of the protons have been replaced bymetal ions.
 3. The process as claimed in claim 1 or 2, wherein from 50to 80% of the protons have been replaced by metal ions.
 4. The processas claimed in any of claims 1 to 3, wherein the metal ions are alkalimetal ions.
 5. The process as claimed in any of claims 1 to 4, whereinthe metal ions are sodium ions.
 6. The process as claimed in any ofclaims 1 to 5, wherein the reaction is carried out at from 20 to 120° C.7. The process as claimed in claim 6, wherein the reaction is carriedout at from 40 to 100° C.
 8. The process as claimed in any of claims 1to 7, wherein the reaction is carried out in a liquid phase in thepressure range from 5 to 40 bar.
 9. The use of the diisobutene preparedas claimed in any of claims 1 to 8 for preparing 3,5,5-trimethylhexanoicacid by hydroformylation and subsequent oxidation.
 10. The use of theisobutene prepared as claimed in any of claims 1 to 8 for preparing3,5,5-trimethylhexanol by hydroformylation and subsequent hydrogenation.11. The use of the diisobutane prepared as claimed in any of claims 1 to8 as alkylating agent for phenols and aromatic amines.